1. Field of the Invention
The present invention relates to novel polymer gel electrolytes prepared by solidifying a high-boiling fire-retarding plasticizer, and to secondary cells and electrical double-layer capacitors made using such polymer gel electrolytes.
2. Prior Art
Non-aqueous electrolyte solution-based secondary cells such as lithium secondary cells have attracted much attention in recent years on account of their high voltage and high energy density. The solvent used in such non-aqueous electrolyte solutions is a mixed solvent composed of a cyclic carbonate or lactone having a high dielectric constant and a high viscosity, such as propylene carbonate (PC), ethylene carbonate (EC) or γ-butyrolactone (GBL), in combination with a low-viscosity acyclic carbonate, such as dimethyl carbonate (DMC) or diethyl carbonate (DEC), or a low-viscosity ether such as 1,2-dimethoxyethane (DME), diglyme or dioxolane.
Further improvements in safety, such as fire retardance and non-flammability, will be needed to attain even higher levels of energy density and output density. Yet, such goals have been very difficult to achieve with existing low-flash-point flammable non-aqueous electrolyte solutions.
A number of solutions have already been proposed, including methods involving the addition of a flame-retarding phosphate to the electrolyte solution (JP-A 4-184870 and JP-A 8-88023) and methods involving the addition of an alkylene carbonate or a halogenated alkylene carbonate to the electrolyte solution (JP-A 9-306542), JP-A 9-312171, JP-A 10-251401, JP-A 12-260467). However, in batteries and electrical double-layer capacitors made with such electrolytes, the supporting electrolyte salt has a poor solubility, resulting in a low ionic conductivity. In addition, undesirable effects such as fluid leakage to the exterior and leaching out of the electrode material tend to arise, compromising the long-term reliability of the battery or capacitor.
By contrast, batteries and electrical double-layer capacitors made with solid electrolytes are free of such problems and offer the additional advantage of being easy to form into a thin shape.
U.S. Pat. No. 4,792,504 describes a method for improving ionic conductivity by using a polymer gel electrolyte prepared by impregnating polyethylene oxide with an electrolyte solution composed of a metal salt and an aprotic solvent, but this polymer gel electrolyte does not have a sufficient ionic conductivity or film strength. To overcome this problem, JP-A 6-187822 discloses an ion-conductive solid polymer electrolyte made with a complex composed of an electrolyte and a polymer that is prepared from a mixture of oxyalkylene group-bearing (meth)acrylate monomers having urethane linkages.
However, a system prepared by gelating a non-aqueous electrolyte which is flammable and has a relatively low flash point is subject to the same service temperature range limitations due to solvent vaporization and gelation as solutions of the non-aqueous electrolyte. In addition, there are problems with polymer gel electrolyte production and with the safety of batteries and electrical double-layer capacitors in which such electrolytes are used.